GENEVA, Sept 11 (Reuters) - The United Nations human rights chief warned on Monday that the government of Venezuelan President Nicolas Maduro may move to further crush democratic institutions and that crimes against humanity may have already been committed by his security forces.

Venezuela has been convulsed by months of near-daily demonstrations against the leftist president who critics say has plunged the oil-rich country into the worst economic crisis in its history and is turning it into a dictatorship.

“There is a very real danger that tensions will further escalate, with the government crushing democratic institutions and critical voices,” Zeid Ra’ad al Hussein told the U.N. Human Rights Council.

The government was using criminal proceedings against opposition leaders, arbitrary detentions, excessive use of force and ill-treatment of detainees, which in some cases amounted to torture, he said.

Last month, his office said that Venezuela’s security forces had committed extensive and apparently deliberate human rights violations in crushing anti-government protests and that democracy was “barely alive”.

“My investigation suggests the possibility that crimes against humanity may have been committed, which can only be confirmed by a subsequent criminal investigation,” Zeid said on Monday.

Venezuela’s Foreign Minister Jorge Arreaza Montserrat was due to address the 47-member Geneva forum later in the day, where his country is a member state and enjoys strong support from countries including Cuba and Iran. (Editing by Tom Miles)